1. Field of the Invention
The present invention relates generally to a hydraulic control system for an automatic transmission of an automotive vehicle. More particularly, the invention is directed to a hydraulic control system which serves to adjust a level of fluid pressure to actuate frictional elements of an automatic transmission during exhaust braking operation for securing tight engagement of the frictional elements.
2. Description of The Prior Art
It is current practice to increase engine braking effects by using a control valve which closes an engine exhaust passage to restrict exhaust gas flow, causing internal resistance of the engine against engine rotation to increase. Additionally, for automatically changing gear, an automatic transmission utilizes a power train including gears and frictional elements such as clutches and brakes. When changing gear, the preselected frictional elements are engaged or disengaged by hydraulic pressure. The hydraulic pressure for controlling the frictional elements is generally provided with fluid pressure produced by an oil pump which is in turn modified to line pressure through a primary regulator valve according to vehicle speed or a throttle valve opening angle.
During exhaust braking operation, a relatively great driving force is transmitted to the automatic transmission from road wheels which may rotate the engine in a reverse direction with the result that the hydraulic pressure for changing gear becomes insufficient for securing tight engagement of the frictional elements, causing the frictional elements to slip undesirably. This results in the durability of the frictional elements being degraded.
For avoiding the above drawback, Japanese Patent First Publication No. 62-61843 proposes a hydraulic control system for an automatic transmission which elevates hydraulic pressure for actuating frictional elements by a certain degree during exhaust braking operation.
However, the above prior art hydraulic control system for an automatic transmission is adapted for increasing a hydraulic pressure level uniformly when changing a gear ratio regardless of the pay load of the vehicle. Therefore, while the durability of the frictional elements may be improved, the hydraulic pressure tends to be higher than necessary for actuating the frictional elements when the vehicle is traveling with a relatively light load, causing large scale shock to occur upon engagement of the frictional elements. This gives an unpleasant feeling to vehicle passengers.